A vacuum brake booster is a component used on motor vehicles in a braking system to provide assistance to a driver by decreasing their needed braking effort to brake the vehicle. A vacuum brake booster used in a braking system may also be commonly referred to as a vacuum servo, a vacuum booster, a brake booster, or simply as a booster. A vacuum booster uses a stored vacuum to increase a braking force applied by a driver to the brake pedal before applying the transferred force to a brake master cylinder. The vacuum is typically generated in one of two distinct methods, dependent on the type of internal combustion engine, or other motive force (as in electric vehicles). In vehicles that have a naturally aspirated gasoline engines, the intake manifold is typically utilized, whereas in fully electric vehicles or vehicles with a turbo charger or diesel engines, a separate vacuum pump may often be used. In the case of a naturally aspirated gasoline engine, the vacuum booster is typically in fluid connection with cylinders of an engine and the vacuum is pulled during the engine pistons' intake strokes. The fluid connection is traditionally provided along semi-rigid plastic lines and is typically stored in the booster by using a check-valve. A check-valve, clack-valve, non-return valve or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction, in this case toward the engine.
Vacuum boosters may also utilize a vacuum enhancer to pull a deeper vacuum within the booster than available directly from an intake manifold of an engine. A vacuum enhancer may have an ejector disposed between an inlet-duct and an intake manifold, bypassing a throttle, to allow a rush of fluid to provide a venturi effect and enhance the vacuum pulled within the booster to a deeper level. Conventional brake vacuum enhancers use a small size venturi which is always open. With these devices, the rate at which vacuum is generated by these small size venturis is very slow and continuously provides unmetered air to the engine which is not desirable.
An example of a vacuum enhancer with a closeable ejector may be seen in U.S. Pat. No. 6,035,881 to Emmerich et al, which is incorporated herein by reference.
Emmerich et al. relies on the differential pressure between the vacuum booster fitting and the intake manifold to open the ejector flow valve. The intake manifold is not at full atmospheric pressure and the intake manifold pressure varies as the air filter gets dirty. As well, if the vehicle has an intake manifold turbo charger and it spools up and pulls a vacuum onto the manifold, the pressure differential will vary even more. Varying pressure differentials to open and close the ejector path at differing times result in inconsistent performance of the device.
Emmerich et al. also relies on flexing membranes to open and close the ejector. The traditional materials used in this type of construction are rubber sheet, aramid reinforced rubber, and woven nylon, all of which may have varying stiffness changes as a function of temperature. Stiffness changes to the membrane due to age may also result in changes to the pressure differential to open and close the valve. For example, new devices in cold areas will require higher vacuum levels to open when compared to old devices operated in hot climate conditions. Varying stiffness of the membranes also results in inconsistent performance of the device